The object of ground proximity warning systems is to prevent aviation accidents in which an aircraft that is still operable crashes into the ground, accidents known in the technical literature under the acronym CFIT, “Controlled Flight Into Terrain”.
The first ground proximity warning systems known under the acronym GPWS did not show threatening relief or obstacles on a map, since they only took into account the aircraft's flight conditions. This is where the alert functions are in so-called “reactive” mode. As they posed a problem of adjusting their sensitivity, calling for a compromise between being triggered in time on each real risk of collision with the ground and a minimum of false alarms, it was quickly sought to improve them by adding to the information taken into account, navigation data and relief maps extracted from onboard topographical databases or those accessible from the aircraft in flight. Hence it is that ground proximity warning systems called TAWS appeared (acronym for “Terrain Awareness Warning System) which in addition to the usual GPWS functions, fulfilled an additional function of predictive alerting of risks of collision with the relief or with ground obstacles consisting in alerting the crew of the aircraft when the short-term predictable path of the aircraft might meet the ground or an obstacle on the ground. This is where the alert functions are in so-called “predictive” mode.
One of the criteria that can be used to judge the quality of a TAWS lies in its nuisance level. A nuisance alert is one which is generated by the TAWS while the aircraft is in a situation such that the operational margins are respected.
Most nuisances observed during the use of a TAWS originate from the so-called “reactive” modes. More precisely, there is, for example, an alert detection mode that consists in comparing the aircraft's height-to-rate-of-descent combination with a chart. There is also a second alert detection mode that consists in comparing height and height variation. An alert is generated for each of these modes when the combination of the two input parameters is considered abnormal. These two modes are therefore based, amongst other things, on the use of a radar altimeter which measures the height separating the aircraft from the relief (or obstacle) located below it.
From the type of mission carried out (transporting casualties from one hospital to another, VIP transport, traffic monitoring, police mission), the helicopter is an aircraft that regularly flies around in an urban area. When flying over dwellings or building structures, the radar altimeter therefore regularly observes large variations in the aircraft's radio altitude over very brief periods of time. These abrupt variations may suffice to trigger a reactive alert while the operational margins are quite satisfactory. These alerts are therefore regarded as nuisances by the crew who may be tempted to deactivate the TAWS system, thus being deprived of the safety net offered by the function.
Reactive modes do not take into account the nature of the environment through which the aircraft is flying. Reactive alerts will be generated in the same way for speed and radio altitude conditions and for a given relief, whether the aircraft is in the middle of a densely populated area in terms of obstacles (e.g. New York City) or whether it is in the middle of a desert area (e.g. the Sahara). Moreover, reactive modes use the data from the aircraft's sensors directly. These are more or less filtered according to the carrier and the type of sensor. This filtering is generally the same for all the systems using the datum generated.
Patent document 0701796 is known, disclosing a method for reducing nuisance alerts for anti-collision with obstacles for predictive modes. This method consists in calculating a weighting coefficient of the duration of a protection envelope in front of the aircraft as a function of obstacle density.